1. Field of the Invention
The invention relates to an electronic circuit, particularly to a crystal oscillator.
2. Description of the Related Art
The traditional crystal oscillator, as shown in FIG. 1, usually uses a inverter 102 to generate an oscillating signal. The trans-conductance Gm of the inverter isGm=Kn×(Wn/Ln)×(0.5Vdd−Vtn)+Kp×(Wp/Lp)×(0.5Vdd−Vtp)  equation 1
In order to ensure that with all kinds of crystal Xtal, and a variety of circuit board load conditions, the crystal oscillator still be able to maintain a stable output. The trans-conductance Gm of the inverter is usually a large value. For example, trans-conductance Gm is greater than 5 mA/V. But in order to increase the inverter trans-conductance Gm in the traditional crystal oscillator, the aspect ratio (Wn/Ln, Wp/Lp) of the two NMOS and PMOS transistors of the inverter are increased respectively. And this makes the inverter to increase the power consumption. The power consumption of the inverter is:I=0.5×Kn×(Wn/Ln)×(Vdd−Vtn)2; when input=Vdd I=0.5×Kp×(Wp/Lp)×(Vdd−Vtn)2; when input=Vss  equation 2
Therefore, it can't both increase trans-conductance Gm and reduce power consumption in the traditional crystal oscillator.
Furthermore, with the inverter consume greater power, it is easier to interference other circuits, specifically for those noise-sensitive circuits, such as analog circuits, radio frequency circuits. And, with the technology develops continuously to the high frequency, high speed, low operating voltage, and low power consumption. For example, digital-to-analog converter (DAC); another example, Ethernet develops from 10M through 100M, 1 G, 10 G, 40 G, and other more high speed development; another example, integrated circuit process develops from 0.5 process through 0.35, 0.25, 0.18, 0.15, and 0.09 processes, that shows the important and necessary of the components with low power consumption, the less interference with other circuits.